Scaling is a problem that is commonly encountered wherever minerals are dissolved in water. It is a serious problem in aqueous systems, such as cooling water systems; water purification systems; boilers; desalination plants; gas scrubbing; steam generators; dishware and hard surface cleaning; gas and oil production processes, such as topside and downhole applications, sub-sea flow lines, umbilical lines, capillary strings, gravel packs, functional fluids used in oil production; paper processing; sugar refining; mining (heap leaching); and geothermal wells.
In the case of industrial water treatment, such as cooling systems and boilers, the formation of scale is dependent on the characteristics of the water, such as, hardness, pH, temperature and the concentration. In these systems stability to oxidizing biocides, such as bleach (sodium hypochlorite/hypobromite), is very important. In addition, because the cooling water reaches the environment, generally via streams, rivers, oceans and other waterways, the environmental profile of scale inhibitors has becoming increasingly regulated.
In oilfield production, water is heavily used in the oil extraction systems. For example, the water is injected under pressure into an oil reservoir that contains what is generally known as formation water. The pressurized water forces subsurface oil and formation water into nearby production wells. Formation water is usually hard water that contains various scale forming polyvalent metal cations, such as barium, calcium and magnesium. Under certain conditions these metal ions form insoluble salt deposits or scale in processing equipment n.
For offshore oil or gas production, seawater is often used as the injection water for extracting oil. Seawater contains sulfate and carbonate anions. When seawater and formation water come together, the sulfate or carbonate for example from seawater reacts with barium and/or calcium formation water to form insoluble salts, such as barium sulfate, calcium sulfate and calcium carbonate. The insoluble salt deposits or scale readily form on pipes and other production equipment. Sometimes, the formation water may also contain radioactive materials that are incorporated in the scale.
Scale formation can be mitigated or controlled by several methods. For example, calcium carbonate scale can be treated by adding an acid or CO2. However, acid addition can cause increased corrosion and large quantities of acid may be needed to lower the pH sufficiently. As such, this process may be economically unattractive. Equipment can be periodically chemically or mechanically cleaned to remove scale.
The formation of deposits can be prevented by the use of chemical compounds referred to as “scale inhibitors.” Scale inhibitors are substances that significantly reduce the formation of scale, partly by inhibiting crystallization and/or retarding the growth of scale forming minerals when applied in sub-stoichiometric amounts. Currently, scale is often treated by the addition of sub stoichiometric levels of water soluble organic scale inhibitors in the 1-500 ppm dosage range. These scale inhibitors are often referred to as threshold scale inhibitors, i.e. there is a threshold dose level below which they do not inhibit scale formation. This limit is often referred to as the minimum inhibitor concentration (MIC).
In order to deliver a required scale inhibitor into an oil production well a squeeze treatment is often performed. This is a method in which a scale inhibitor solution is pumped directly into a formation, often via the production well. An over flush of sea water is used to push the inhibitor further into the formation and into the region around a production well. When oil is subsequently produced, scale inhibitor is released into the water which is produced with the oil.
Clearly some degree of adsorption onto the formation rocks is required so that the inhibitor is released slowly. Inhibitors used in oilfield applications must be stable downhole to high temperatures and they must be compatible with seawater and the formation water present in the oil formation. The lifetime of the squeeze treatment is the time that it takes for the inhibitor in the produced water to drop below the MIC. Any produced water is either re-injected or eventually reaches the environment, so the environmental profile of the inhibitor is very important and there is a growing need for biodegradable scale inhibitors.
Well known organic scale inhibitors, used in multiple applications, are typically polymeric e.g. polyacrylates, polymaleates, poly sulfonates, or small molecules, such as phosphonates and bisphosphonates.

These products are not readily biodegradable and as environmental concerns become more important chemicals such as scale inhibitors, are increasingly scrutinized and legislated. Currently, preferred phosphonates and bisphosphonates include PBTC, HEDP, AMP and diethylenetriamine methylene pentaphosphonate.
One drawback to the polymers and phosphonates described hereinabove is that they are not readily biodegradable. They generally are less than 60% biodegradable in 30 days. Furthermore, the currently available HEDP and AMP phosphonates based products are not stable against oxidizing biocides such as bleach.
There have been several attempts to provide improved “eco-friendly” or environmentally safe biodegradable scale inhibitors. Well known industrial eco-friendly scale inhibitors, include various polyaspartates and inulin.
Bain, Fan, Fan and Ross describe a thermal polyaspartate for use as a biodegradable scale inhibitor. (D. Bain, G. Fan, J. Fan and R. Ross “Scale and Corrosion Inhibition by Thermal Polyaspartates”, Corrosion/99 paper 120, NACE 1999). However, the proposed thermal polyaspartate does not perform well, particularly under severe scaling conditions such as that of production wells.
Kohler, Bazin, Zaitoun and Johnson propose the use of certain Polyaspartates or Carboxy Methyl Inulin (CMI) as a biodegradable scale inhibitor. (N. Kohler, B. Bazin, A. Zaitoun, T. Johnson “Green Inhibitors for Squeeze Treatments: A Promising Alternative”, Corrosion/04 paper 4537, NACE 2004) However, the proposed CMI and polyaspartates do not work as well as other non-biodegradable inhibitors and neither product is fully biodegradable.
Overall, the known polyaspartates and inulin are not highly active and require huge doses to perform at an acceptable level. Accordingly these known polyaspartates and inulin are not very cost effective when compared to current non-biodegradable products. Thus, there is a widely-recognized need in the oilfield and water treatment industries for new cost effective biodegradable scale inhibitors, capable of operating under severe scaling conditions.
A common practical problem is that such biodegradable inhibitors do not exhibit sufficient thermal stability in use. Even where scale inhibitors are highly cost-effective, further constraints are imposed by the need for them to be thermally and hydrolytically stable, safe for operators to use, environmental friendly and compatible with high levels of scaling cations and other chemicals that may be added to the system, such as corrosion inhibitors, both non-oxidizing and oxidizing biocides and demulsifiers.